1. Field of the Invention
The present invention relates to a sprocket wheel and a lens-fitted photo film unit with the same. More particularly, the present invention relates to a sprocket wheel for use with photo film which has two perforations per one frame, and a lens-fitted photo film unit with the same sprocket wheel.
2. Description Related to the Prior Art
A photo film of the IX240 type according to the "Advanced Photo System (trademark)" is known. This type of photo film is contained in a resinous cassette different from a photo film cassette of the 135 type, and has two perforations per frame. Compact cameras or lens-fitted photo film units, such as Fujicolor Quicksnap Super Slim (trade name, manufactured by Fuji Photo Film Co., Ltd.), are widely sold as products in which the photo film of this type is contained.
A photo film winding device is incorporated in the lens-fitted photo film unit. The photo film winding device includes a sprocket wheel, in which two pairs of teeth are disposed thereabout. Among the teeth, one pair of teeth is positioned in a rotationally symmetrical manner from another pair of teeth. In the photo film, the perforations are regular but intermittent. There occurs a situation where a relatively long section between two of the teeth is confronted with the photo film, and the teeth are not meshed with any of the perforations. In view of this, the photo film winding device includes a spring of a plate shape. A cam member is secured coaxially with the sprocket wheel, and is biased rotationally by the spring, so that the spring rotates the sprocket wheel in a winding direction. Thus a succeeding one of the teeth of the sprocket wheel can come to a position in mesh with one of the perforations in the photo film. With the rotary cam member rotated, a shutter device is charged, and a counter wheel is rotationally set for indicating the number of remaining available frames.
In the photo film winding device, the rotary cam member is rotationally biased by the spring with great biasing force, so that the photo film winding device has a shortcoming in that a photo film winder wheel cannot be rotated lightly by manual operation. One option is conceived to reduce the biasing force of the spring. However another problem arises in that the sprocket wheel cannot be rotated to a sufficient extent. It is likely that a succeeding one of the teeth of the sprocket wheel fail to come to a suitable position contacting the photo film, and will fail to be in mesh with one of the perforations. If a user uses the lens-fitted photo film unit in an environment with much minute dust, for example on the seashore with sand on a windy day, dust is likely to enter a top opening or slit about a shutter release button, and remain between the rotary cam member and the spring, so that the sprocket wheel does not properly rotate. The photo film is not acceptably fed by one frame.